This invention relates to brake testers for vehicles, and more particularly to plate brake testers particularly suited for testing the brakes of multi-axle vehicles.
Faulty or inadequate brakes are a significant cause of vehicle accidents. Moreover, the faulty condition of the brakes is often not apparent during routine use of the brakes. The operator of the vehicle is usually not aware, until it is too late, that the vehicle's brakes are deficient.
Furthermore, many brake defects are not readily apparent during a visual inspection of the brakes. Only a test simulating actual stopping conditions can detect many brake defects.
When the brakes of a vehicle are applied, a retarding force is generated between the tire and the surface on which the tire is riding. When this force becomes greater than the weight on that wheel multiplied by the coefficient of friction between the tire and the surface, the wheel will begin to lock up and stop rolling. The retarding force of a wheel just before lockup is greater than the retarding force of the same wheel just after lockup. In addition, a locked wheel loses its ability to maintain lateral forces, which makes handling very difficult.
Since the maximum usable force of a brake is related to the weight on that wheel, vehicle designers adjust the braking system so that the brake force distributions coincide with the vehicle weight distribution. When a vehicle is decelerating, the forces acting on the center of gravity of the vehicle cause a weight shift. The "dynamic weight" appearing on each wheel/tire assembly is, therefore, different from the "static weight" when the vehicle is at rest.
Apparatus for testing brake performance are available, but they could be improved. For example, the results of the brake test under actual stopping conditions can depend upon the make and model of the vehicle, the actual deceleration applied to the vehicle during the test, and the static and dynamic weight distribution of the vehicle during the test. Apparatus which would take into account all these factors have heretofore been considered too complicated or too slow.
Present brake testing systems compensate for the various factors by setting broader than necessary ranges for the acceptability of the measured braking forces. In some cases these broad ranges allow vehicle brakes to "pass" which, for that particular vehicle and deceleration, should have failed.
U.S Pat. No. 5,083,456, assigned to the assignee of the present application, addresses many of these drawbacks, in part by estimating the dynamic weight distribution of the vehicle under test. Although this works extremely well for automobiles, it is less satisfactory for multi-axle vehicles such as tractor-trailer trucks where such estimation is more difficult and less accurate.
With respect to trucks, truck loading makes a significant difference in the apparent acceptability of brakes. A fully loaded truck responds very differently than a lightly loaded one. A test designed for fully loaded trucks can, therefore, fail to accurately indicate the actual condition of the brakes when the truck is tested during light load conditions.